The present invention relates to the measurement of parameters of a video system, and more particularly to a recursive noise reduction filter for video components that reduces the effective noise bandwidth of the video components without degrading the signal spectrum to enhance the ability to measure differential errors of the video system.
In a video system there are two luminance dependent chrominance signal distortions in a video signal: differential gain and differential phase. Differential gain is a change in color subcarrier amplitude as a function of luminance so that in the reproduced color picture the saturation is distorted in the areas between the light and dark portions of the scene. Differential phase is a phase modulation of the chrominance signal by the luminance signal so that in the reproduced color picture the hue varies with scene brightness. Differential gain and differential phase may occur separately or together and are caused by amplitude nonlinearity and time delay that are not independent of the signal level. To measure these parameters a modulated ramp or staircase video test signal that requires a relatively large horizontal bandwidth is input to the video system, and the chrominance demodulated signal from a resulting video signal output from the video system is amplified by a very large gain factor, generally in excess of thirty times. The measurement of these parameters in the presence of noise is difficult primarily for two reasons. First the signal to noise ratio (S/N) suffers due to the high gain of the demodulators to obtain the desired resolution needed for most measurements since differential gain and phase errors are generally small; and second a relatively large horizontal noise bandwidth is required to pass the test signal without distorting the measurement, further degrading the S/N. An example of a differential gain/phase display having significant noise and small errors is illustrated in FIG. 1 where the noise effectively masks the signal to be measured.
The differential gain and differential phase measurements are even more difficult where the video system is a video tape recorder (VTR) with a typical S/N of 45 to 50 dB. The measurement difficulty also is increased when the line select mode is used since the test pattern, the vertical interval test signal (VITS), occurs at only one line per frame, i.e., at 30 Hz, rather than at every line (full field), i.e., at 15.734 kHz. The full field test signal allows an operator to integrate the Gaussian distribution of the noise with the eye. However, in line select mode the human eye is unable to integrate sufficient samples to observe the Gaussian distribution. Other differential measurements include luminance to chrominance group delay in response to a modulated sine-squared test signal.
Therefore what is desired is a method of measuring differential error characteristics of a video system in response to a vertically coherent video test signal with enhanced accuracy in the presence of noise by reducing the noise bandwidth without degrading the signal spectrum.